The performance of covert illuminators is presently constrained by laser sources which either emit with high brightness in a wavelength region beyond the range of high quantum efficiency low noise imaging devices or by sources which exhibit inherently low brightness but operate in a wavelength region of high sensitivity detectors.
In general, active night viewing systems employ either GaAs illuminators coupled with high quantum efficiency-low noise imaging photocathode surfaces (S-20, GaAs) or Nd: YAG illuminators coupled with low quantum efficiency photocathode surfaces (S-1) which have inherently high dark current. In the former case, the available range of illumination is limited by the inherently large beam divergence of the laser diode of the transmitter. In the latter case system performance is limited by the poor sensitivity of receivers in this wavelength region (1.06 microns). Enormous improvement of the long range viewing capability can be obtained by utilizing for the transmitter an optically pumped solid state laser operating in a spectral region bounded on the long wavelength side by the wavelength limit of good receiver performance and on the short wavelength side by the wavelength limits determined by the sensitivity of the human eye. This region presently extends from approximately 0.8 to 0.9 microns.